Seleninic acids synthesis

In an unusual modified chromone synthesis, enaminones formally derived from o-hydroxyacetophenone react with selenium oxychloride prior to cyclisation resulting in the formation of Se-bridged bischromones. Various reactions at Se are described, including fission of an Se-Se bond, leading to chromone-3-seleninic acid (95JHC43). 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

Elimination reactions leading to olefins are usually performed on the corresponding selenoxides 3 9,1 12) (Scheme 2 a). These are often unstable and decompose at room temperature to olefins and selenenic acid (further oxidized to the more stable seleninic acid by excess of oxidant). Hydrogen peroxide in water-THF, ozone and further treatment with an amine or tert-butyl hydroperoxide without or with alumina proved to be the method of choice for such a synthesis of olefins. The reaction is reminiscent of the one already described with aminoxides or sulfoxides 22) and occurs via a syn elimination of the seleninyl moiety and the hydrogen attached to the P-carbon atom. However it takes place under smoother conditions. 

Since seleninic acids and their anhydrides are much more stable than the corresponding selenenic compounds, they can be employed as starting materials for the synthesis of selenenic acids and their anhydrides. Reduction of seleninic acids and their anhydrides with appropriate reducing reagents such as hydrazines [35], hypophosphorous acid (H3PO2) [36], and thiols [37] is a useful alternative method for the synfhesis of selenenic acids and their anhydrides. 

Methods of synthesis of seleninic acids are based on oxidation of diselenides (or selenocyanates) with HNOj [38k, 42[ or H2O2 [23a, 42d, 43[. Seleninic anhydrides are prepared by dehydration of seleninic acids [42 e, 44[ or by oxidation of diselenides with ozone (Scheme 15.14) [44, 45]. 

Excess of 30% H2O2 added to a soln. of 2,4,6-trimethylbenzaldehyde and a little bis(2,4-dinitrophenyl) diselenide in dichloromethane, stirred at room temp, for 28 h, the catalyst filtered off, the crude intermediate dissolved in methanol, a soln. of KOH in the same solvent added in 3-4 portions with water-cooling, and allowed to stand for at least 1 h at room temp. - 2,4,6-trimethylphenol. Y 91%. The method is particularly useful for substrates with 2 or more alkoxy groups or with alkyl groups, acid-sensitive substituents remaining intact. F.e. and with diselenides, seleninic acids, or Se02 as catalyst s. L. Syper, Synthesis 1989, 167-72. 

Important uses of selenenic and seleninic acids and anhydrides (see also the later sections) have been established, in the synthesis of )ff-silyloxy-a-(phenyl-seleno)aldehydes from allyl silyl ethers (Scheme 3) and in the high-yield syntheses of 2-acetoxyalkyl phenyl selenides [using PhSe(0)OH and AcOH j and... 

Treatment of the isomeric dihydroselenines 93 containing electron-withdrawing groups in both the 2- and 6-positions with w-chloroperbenzoic acid also leads to formation of Pummerer-type oxidation products, the 4H-selenines 94 and the corresponding w-chlorobenzoate ester by-products (Equation 39) <1999J(P1)1155>. The esters could be converted to the corresponding 477-selenines by treatment with polyphosphoric acid trimethylsilyl ester (PPSE), a useful reagent for eliminations under neutral conditions. These 4/7-selenines were key intermediates in the synthesis of selenabenzenes. 

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